But how come Harry knows so much about Tegan herself? Is it really just guesswork, as he says? Is Harry right to be concerned about the fairground's fortune teller, who claims to have supernatural skills? Both he and the Doctor suspect an alien influence they know of old.

What neither the Doctor nor his friends realise is that, somewhere in the shadows, a far more sinister and familiar presence is lying in wait for them...

Tegan is suitably impressed, and thrilled to meet the renowned escape artist and illusionist. The reference is lost on Nyssa and Adric, but even they are caught up in his charisma; and when he knows more than he should about Tegan, they are intrigued. Houdini plays it off, however, and diverts to another matter: He has come to the fair to debunk its fortune teller, who has inexplicable abilities of her own, but he has not been able to do so. Now he wants the Doctor's help.

Something is not right, though. Houdini seems intent on drawing information from the Doctor which he should not have, regarding his own future and the workings of the TARDIS. Before it can be addressed, however, the group is split up; Houdini and the Doctor go to check out the fortune teller, and the three companions find themselves inside the fair's menagerie.

The Doctor and Houdini don't find the fortune teller, but they do find something else—her crystal ball. It proves to be no ordinary prop. The Doctor recognises it, and refers Harry back to a previous adventure they shared (in the Doctor's first life, along with Ben Jackson and Polly Wright) in which they encountered the Ovids, beings of pure thought. The Ovids used crystal spheres to communicate and influence the world; spheres just like this one. And this one is already in use.

In the menagerie, the companions are menaced by animals that have been released from their cages, including a tiger. Adric runs, leading the tiger away from the others, giving them time to try to escape. Tegan shouts for the Doctor, but to no avail. Adric is rescued, however, by a group of fairground workers and performers; but it is short-lived, as it becomes clear that they are under some kind of mind control. They escort him to a theatre on the grounds. Meanwhile, Nyssa also has an odd encounter, with a man wearing the face of her lost father: The Master. He takes hypnotic control of her, and forces her to abduct Tegan and bring her to the theatre as well.

The Doctor tries to make telepathic contact with the sphere, and is successful, though it hurts him. As he is about to make progress, he is interrupted by the image and voice of a young man wearing a bow tie—the Eleventh Doctor. He tells the Fifth Doctor that he will soon be tempted to destroy the sphere, but he must not. Instead, he must return it to its rightful owners, the Ovids. At the same time, he is interrupted by Tegan's scream, which the sphere has also picked up; he realises that they are in danger, and he has failed to help them.

The Doctor and Houdini hurry toward the theatre; but Houdini attacks him and takes him captive. He places the Doctor inside a crate that is intended to be used in his famous underwater escape trick, and tells him that he has met an old friend of the Doctor, who revealed to him that the Doctor has manipulated him and withheld information. It's all a lie, of course; and the Doctor recognises the source as the Master. Nevertheless, in an effort to persuade the Doctor to tell him more, Houdini pushes the crate into the water... and watches as the Doctor fails to escape.

On the theatre's stage, Adric is seized by the Master, but he cannot see him. In the struggle, he sees a mirror propped up on the stage, and realises that the Master is visible in the mirror, and their reflections are struggling. It defies science, but there's little time to think about it, as he struggles to break free. Tegan intervenes, and sees the situation, and starts to throw a chair to break the mirror; Adric shouts at her to stop, not knowing what effect it may have on him. Instead, she throws the chair at the spotlight that is trained on the mirror, creating the reflections; it breaks, and the Master vanishes.

All is not saved, though. Enraged, the Master starts to use the Ovid sphere to release massive amounts of electricity through the fairground, setting things on fire and destroying buildings. Houdini catches up with the companions as they try to escape, but they are cut off. Suddenly the Doctor returns, and reveals that he picked Harry's pockets for his lockpicking kit, using that—and his Time Lord ability to bypass respiration—to escape the trap. He had suspected Harry was not himself all along. They make their way back to the fortune teller's booth, the Doctor explaining that the Master was never corporeally there at all; he seems to be still trapped in the collapsing dimension where they last saw him. Instead he was using the sphere to exert his will remotely, and even project himself. He locates the sphere, and is tempted to destroy it—just as promised—but instead, he makes contact with it to try to soothe it and end the energy discharge. It isn't enough, however, and Tegan joins the link, adding her own thoughts to bring the crisis to an end.

As the smoke clears, the fairground workers awaken from their trance, having no memory of the last twelve hours. Houdini convinces them they have been unwitting participants in an experimental new act, which seems to satisfy them. He again attempts to persuade the Doctor to let him see inside the TARDIS, but is gently rebuffed, and admits that it's better to make his future than to know it. As the Doctor and companions leave, Tegan again asks to be taken to Heathrow, but the Doctor tells her they must first make a stop: to return the sphere to the Ovids.